1. Field of Invention
The present invention relates generally to biased transfer rollers for high speed xerographic printing, and more particularly, to biased transfer rollers with commutated longitudinal electrodes embedded below the surface of the roller to control pre-nip and post-nip fields for image transfer.
2. Description of Related Art
Typically, electrostatic imaging and printing processes are comprised of several distinct stages. These stages may generally be described as (1) charging, (2) imaging, (3) exposing, (4) developing, (5) transferring, (6) fusing, and (7) cleaning. In the charging stage, a uniform electrical charge is deposited on the surface of a photoreceptor so as to electrostatically sensitize the surface. Imaging converts the original image into a projected image exposed upon the sensitized photoreceptor surface. An electrostatic latent image is thus recorded on the photoreceptor surface corresponding to the original image. Development of the electrostatic latent image occurs when charged toner particles are brought into contact with this electrostatic latent image. The charged toner particles will be attracted to the charged regions of the photoreceptor surface that correspond to the electrostatic latent image. In the case of a single step transfer process, the photoreceptor surface with the electrostatically attracted toner particles is then brought into contact with an image receiving surface i.e., paper or other similar substrate. The toner particles are imparted to the image receiving surface by a transferring process wherein an electrostatic field attracts the toner particles towards the image receiving surface causing the toner particles to adhere to the image receiving surface rather than to the photoreceptor. The toner particles then fuse into the image receiving surface by a process of melting and/or pressing. The process is completed when the remaining toner particles are removed from the photoreceptor surface by a cleaning apparatus.
Transferring the toner particles from the photoreceptor surface to the image receiving surface of the substrate is usually performed by applying an electrostatic force field in the transfer nip region sufficient enough to overcome the adhesion force between the toner particles and the photoreceptor surface. If the applied force field is sufficient, the toner particles will move from the photoreceptor surface to the image receiving surface.
The area between the photoreceptor and the image receiving surface may be divided into three distinct regions: the nip region, the pre-nip region, and the post-nip region.
The nip region comprises the point at which the photoreceptor and the image receiving surface come into direct contact. Typically most of the toner particles are transferred to the image receiving surface within the contact nip and at the end of the contact nip, just as the surfaces start to separate. The pre-nip region comprises the region upstream from the nip region. In the pre-nip region, there is an air gap between the photoreceptor and the image receiving surface since the two have not yet come into direct contact. The toner particles are attached to the photoreceptor by adhesion forces, and have not yet come into contact with the image receiving surface. The term xe2x80x9cadhesion forcesxe2x80x9d includes both electrostatic adhesion (e.g., the image force) and non-electrostatic adhesion (e.g., van der Waals forces and capillary forces). The post-nip region is downstream from the nip region. There is also an air gap between the photoreceptor and the image receiving surface in the post-nip region. In this region, the majority of the toner particles typically have been transferred to the image receiving surface and will soon be fused to the image receiving surface.
Precise control over the overall transfer field and the charge on the image receiving surface is desired in each region to ensure the most accurate copy of the original image. The transfer field to attract the toner particles may be highest near the nip region to increase the attraction of the particles away from the photoreceptor. If the field gets too large, however, the transfer efficiency may be reduced because of either the creation of wrong sign toner or an increase in adhesion caused by an induced dipole in the toner particle. Controlling the electric field in the pre-nip region better ensures that the toner particles will not be prematurely attracted away from the photoreceptor to the image receiving surface. Excessive electric fields in the pre-nip region may create gap transfer defects because the toner would transfer prematurely to the image receiving surface introducing undesirable artifacts into the transferred image. Excessive electric fields in the pre-nip region may create wrong sign toner due to air breakdown. The force on the wrong sign toner from the transfer field will tend to increase the attraction of the toner to the photoreceptor. Therefore the toner will not transfer to the image receiving surface. Likewise, the post-nip region also benefits from careful electric field tailoring. Excessive electric fields in the post-nip region may overcharge the transferred toner and deposit damaging positive charge on the photoreceptor. Precise control of the post-nip electric fields can eliminate image disturbances and defects caused by fringe fields and/or uneven arcing between the image receiving surface and either the photoreceptor or the bias transfer roll.
It should thus be seen that a method for precisely tailoring the transfer fields generated in each region is desirable.
The force field applied at the transferring nip region may be generated in several methods. One method, as described in U.S. Pat. No. 2,807,233, positions a transfer corona generator opposite the photoreceptor in the nip region. The transfer corona generator emits ions onto the back of the image receiving surface to cause the toner particles to move onto the image receiving surface. Another method of generating a force field in the transfer nip region comprises a DC charged biased transfer roller or belt rolling along the back of the image receiving surface. When using a biased transfer roller, several different systems are available.
U.S. Pat. No. 3,781,105 discloses the version of the biased transfer roller that is most widely practiced in the xerographic printing industry. The biased transfer roller consists of a relaxable elastomer surrounding a metallic shaft, and does not include any embedded electrodes. The shaft is biased with a constant current high voltage power supply. In principal, partial field tailoring can be achieved by carefully controlling the resistivity of the elastomer, wherein the elastomer must be carefully tuned in order to suppress the pre-nip fields with field tailoring. However, in practice, precisely controlling the elastomer resistivity has not been possible. The resistivity must be controlled within less than a factor of ten (less than an order of magnitude) to ensure successful field tailoring. This is extremely difficult to achieve even when using very expensive elastomers. Part to part variations may exceed this range, and relative humidity can cause the resistivity to shift outside the this range within a given roller. As a result, reliable field tailoring has not been achieved using this method.
The present invention, however, can reliably achieve the desired level of field tailoring with a much wider resistivity latitude for the elastomer. The resistivity latitude if the invention exceeds two orders of magnitude, and relaxable elastomers that can hold this tolerance are easily available.
A fixed transfer block containing spaced and variably biased conductive bars integrally molded into a resistive material to provide tailored image transfer fields is disclosed in U.S. Pat. No. 3,830,589. Transfer rollers containing multiple biased conductors which rotate with the roller are taught in U.S. Pat. No. 3,832,055. U.S. Pat. No. 3,936,174 discloses stationary electrically biased conductive blade-like electrodes inside a thin-walled rotatable outer tube of the biased transfer roller. Each uses the same fundamental method wherein a stationary electrode applies a charge to the surface of the biased transfer roller in a particular transfer region.
The current techniques for creating a transfer field are not adequately tailored for precise control over premature transfer of toner particles from the photoreceptor to the image receiving surface and retransfer.
In view of the foregoing background, it is an object of the present invention to better control the nip, pre-nip and post-nip fields of high speed xerographic printing. Excessive pre-nip fields can generate wrong sign toner and gap transfer defects. Excessive post-nip fields can overcharge the transferred toner and deposit damaging positive charge onto the photoreceptor.
These and other objects of the present invention are achieved by embedding electrodes into a biased transfer roller. Embedded electrodes may be biased such that the electric fields leading into and out of the nip (transfer) region can be easily and precisely controlled to avoid the before-mentioned imaging defects.
The electrodes are embedded onto a biased transfer roller substrate. The electrodes are subsequently surrounded by a conformable semi-conductive layer that can relax the charge accumulated on the surface of the biased transfer roller.
The embedded electrodes may be biased in several different schemes. The electrodes may be grounded in the pre-nip and post-nip regions, but biased in the nip region. All three regions may be biased, or the bias may be varied within each individual region. The bias may even be applied to widely separated electrodes to allow the voltage drop along the semi-conductive surface layer between them to provide the field tailoring. The electrodes far from the nip may be grounded to facilitate the relaxation of charge that has accumulated on the BTR surface.